docs: def get_source_value(self, obj, source, **kwargs):
          """
          Treat ``field`` as a nested sub-Column instance, which explicitly stands in as the object
          to which term coercions and the query type lookup are delegated.
          """
          result = []
          for sub_source in self.expand_source(source):
              # Call super() to get default logic, but send it the 'sub_source'
              sub_result = super(CompoundColumn, self).get_source_value(obj, sub_source, **kwargs)
              result.extend(sub_result)
          return result